Of all the great 17th century metaphysical systems, that of Gottfried Leibniz is undoubtedly the most extravagant. Oscillating between meticulous logic and fantastic speculation, the challenge of appropriately interpreting Leibniz's project is exacerbated by the scarcity of full-length treatises in his corpus. The great majority of Leibniz's philosophical insights are contained in brief essays, prolegomena to unfinished works, and correspondences with his contemporaries, resulting in a remarkably diffuse body of literature. The work of tracing the historical and intellectual development of Leibniz's numerous distinctive concepts is thus as important as it is difficult. In this project, I strive to illuminate at least one aspect of this progression, namely, the issue of Leibniz's mereological thought: how parts relate to wholes in his view. It is well known that the Leibnizian system hinges on causally-isolated, simple substances which Leibniz would eventually dub "monads". However, the question of how the monads relate to extended matter -- whether as parts or in some other fashion -- is significantly more opaque. As my research demonstrates, Leibniz does indeed possess a sophisticated mereological worldview, but the extent to which it lends itself to a single, coherent metaphysical picture may be uncertain. I attempt to expose an apparent disconnect which arises from an intertextual consideration of several Leibnizian discourses, and seek a way of reconciling this inconsistency while remaining faithful to Leibniz's long-term philosophical project.